While the United States is overwhelmingly vaccinated against preventable viruses like measles, mumps and rubella (on account of them coming altogether in one shot), there are certain pockets around the country where vaccination rates are dipping below the 95 percent needed to maintain herd immunity. These under-vaccinated communities, coupled with travelers bringing the measles over from other countries, have resulted in an unusual amount of measles cases -- 644 cases over 23 outbreaks in 2014, and in 2015 to date, 178 cases over four outbreaks. These numbers represent the greatest levels of measles that America has ever seen since measles was first eradicated from the country, in 2000.

The size and scope of the biggest outbreak this year, which links 131 cases to exposure at the Disneyland theme park last December, has focused the nation’s attention like a laser to the tiny communities scattered around the U.S. that have chosen to skip vaccinating their children, without medical justification. In the story below, three infectious disease experts weigh in on what America has learned by turning an ear toward these communities and keeping a wary eye on the growing number of infections.

1. The measles vaccine may be a victim of its own success.

Dr. Walter Orenstein, Associate Director of the Emory Vaccine Center, says a lack of memory about the scourge of measles may be contributing to rising non-vaccination rates in certain pockets of the country. Americans used to be really scared of the measles -- until one day, we weren’t. To illustrate the point, Orenstein related a personal story about the polio vaccine, which he received in 1955, about how insistent his mother was on protecting him from the paralyzing disease.

"I tell a joke; I was in second grade when the Salk polio vaccine was licensed,” said Orenstein over the phone. "I was none too happy to get a shot for something I knew nothing about, but my mother said, ‘Better you should cry than I should cry.’”

"There were fears about these diseases that aren’t here today, in part because vaccines have been victims of their own success,” he continued. “If people aren’t aware of it, they can let down their guard.”

Because measles is so contagious, the virus used to be a near-universal and serious disease in the United States. From 1912 to 1916, there were an average of 5,300 measles deaths per year. And while deaths began going down thanks to better nutrition and improved medical support, in the late 1950s, complications from the disease left an average of 150,000 people with respiratory problems and 4,000 with encephalitis (brain swelling). A 1962 study also found that more than 90 percent of Americans had contracted measles before the age of 15.

That all changed in 1968, when doctors began to distribute the measles vaccine as we know it today. Almost immediately, the measles rate plunged, and then plunged again when public health officials began recommending a second shot of the vaccine. The successful vaccine eventually helped eradicate measles transmission in the U.S. by 2000.

Now the memories of painful measles rashes and potentially deadly complications from the virus are long gone from the public conscious -- and that may be a bad thing. Parents are robbed of an urgency to vaccinate as soon as possible, and doctors have gone so long without treating the disease that they seem to be missing diagnoses, which is what seemed to be the case when members of an Arizona family visited three separate healthcare facilities before anyone diagnosed them with measles.

2. The U.S. needs to help other countries strengthen their immunization programs.

Despite the fact that measles is a completely preventable disease, the virus still infects about 20 million people and kills an estimated 146,000 around the world. Some of the infected make their way to the U.S., where they can encounter pockets of under immunized communities (as they did with the Amish in 2014, or Disneyland visitors in 2014 to 2015). It’s the perfect powder keg for a measles outbreak, and one way to prevent that from happening is to mount a worldwide immunization campaign to eliminate the virus from the world. Indeed, by February of this year, the U.S. had already experienced measles importations from Indonesia, Azerbaijan, Qatar, India and Dubai, according to Dr. Anne Schuchat, Assistant Surgeon General and the director for the CDC's Center for Immunization and Respiratory Diseases.

For FY2015, the U.S. Agency for International Development has given $200 million to Gavi, an international vaccine alliance that has immunization programs in countries like Yemen and Haiti -- up from $75 million in FY 2009. U.S.A.I.D. has also pledged a total of $1 billion from 2015 to 2018, which is estimated to help vaccinate an additional 300 million children and save at least 5 million lives. This funding is key to the U.S.’s continued relative safety from measles, even as non-vaccination may rise in some American communities, explains Michael Osterholm, director of the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota.

“International programs are so important; it’s not just the direct benefits to those countries around the world, but it’s an indirect benefit to us,” said Osterholm to HuffPost. "There’s a vaccine protection halo that we benefit from when we protect those in other countries."

But much more still needs to be done, and failed and fragile states are particularly vulnerable to viral outbreaks. For instance, because of the havoc that the Ebola outbreak wreaked on countries in West Africa, public health officials fear that millions of children are now vulnerable to measles because they’re behind schedule on their vaccinations.

“We have so many situations today where underlying governance issues are so significant that it makes it a challenge to deliver any kind of childhood immunization program,” said Osterholm. "Depending on how you define it, we may have as many as 33 to 36 failed states in this world today, where its very difficult to deliver health care services, including vaccinations.”

3. We’re failing to explain basic scientific concepts in school.

Measles vaccines may be understandably low on the list of priorities for parents in war-torn or fragile states, and in the U.S., under-vaccination used to be a problem concentrated in low-income, poorly educated parents with little access to transportation or services. But one of the stranger paradoxes behind low vaccination rates in some pockets of the U.S. in 2015 is that wealthy, well-educated parents seem to be behind the non-vaccination “movement." Why would these parents take such an anti-science perspective, especially if they’re affluent and college-educated or better?

“It’s a failure of our educational system,” said Orenstein. “I call [unvaccinated children] children of of more highly-schooled, but not highly-educated parents.” This distinction manifests itself in the rational -- yet incorrect -- belief in the causal link between vaccines and autism diagnoses, or the observation that if Unusual Event B happens after Unusual Event A, then A must have caused B. But a single case, or even multiple cases of B coming after A, do not prove a causal link. What’s needed instead, explained Orenstein, is an epidemiological study (a population-based study) to determine if autism rates are higher in vaccinated children vs. unvaccinated children. And epidemiological studies have proven, time and again, that vaccinations do not cause autism.

4. We still don’t understand why people don’t vaccinate themselves and their children.

Scientific evidence in support of vaccines is one thing, but we still don’t understand what motivates a parent in a developed country to forgo vaccines for their children. While it’s easy to ridicule or shame them before dismissing them as a lost cause, key to defeating measles for good will be for researchers to humble themselves and admit they don’t know the slightest thing about what motivates these parents, said Osterholm.

“We have a lot of work to do to figure out really what it is that we know and don’t know, and be humble enough to accept that we don’t know,” said Osterholm. “What motivated people not to get vaccinated, and who are they?” This is a major issue both for the U.S. and some countries in Europe, where anti-vaccination movements are even more, ahem, virulent.

Once we understand the whys, said Orenstein, we could develop stronger programs and tactics to help acknowledge parental concerns while presenting facts to counter the myths. Currently, research on how to change a parent’s mind to accept vaccines is scary and sobering; there’s evidence that tackling the issue from a number of different angles made anti-vaccination parents even more stubborn about their beliefs, according to a 2014 study of 1,800 parents. It’s studies like these that make Osterholm suspect that strict state policies mandating vaccination may actually backfire.

"There’s actually been some recent data showing that the more comprehensive and potentially punitive your efforts are to get people vaccinated, the more people actually reject and fight it, and do whatever they can to get around it,” said Osterholm. "We have to understand that better, because in the end we want to see more people up to date on their vaccinations” -- not just pass new, ineffectual laws.

On the other hand, policy experts have found that the harder a state makes the process of seeking a non-medical exemption to vaccines, the fewer parents pursue that option, reports Mother Jones.

5. Other people (besides non-vaccinators) are claiming medical rights, too.

The United States has always been a country to glorify individual rights, even at the expense of the public good (see: gun laws, or lack thereof). But one good thing that has come out over the debate about mandatory vaccination policies is that other groups of people are beginning to talk about their right to health, just as some non-vaccinating parents hold fast to their right to medical choice.

"The really important conversation started in the U.S. because of this outbreak is that the conversation around vaccination has shifted from personal rights to the consideration of additional rights,” said Dr. Stephen Parodi, an infectious disease physician and associate executive director for the Permanente Medical Group. For instance, while some may claim the right to choose vaccines (or not), others, like the immunocompromised, who can’t get vaccines for medical reasons, and parents of infants who are too young to get vaccinated, are speaking out about their right to herd immunity and protection from preventable viruses.

Carl Krawitt, a father from the anti-vaccination hotspot of Marin, California, petitioned to the school board in January to bar unvaccinated children from the school where his son, six-year-old Rhett Krawitt, goes to school. Why? Because Rhett is a leukemia survivor, and he can’t get a vaccine because his immune system is still recovering from four years of chemotherapy.

"If you choose not to immunize your own child and your own child dies because they get measles, OK, that's your responsibility, that's your choice,” said Krawitt to NPR on Jan. 27. "But if your child gets sick and gets my child sick and my child dies, then ... your action has harmed my child.”

Another dad, pediatrician Tim Jacks of Phoenix, Arizona, wrote a scathing open letter to the mother who chose not to vaccinate her children and ended up exposing his three-year-old daughter to measles when she sought treatment at a hospital. Like Rhett, Jacks’ daughter Maggie also has cancer, and is too immunocompromised to get vaccinated. Jacks also has a 10-month-old son who is too young to get the first measles vaccination -- usually given somewhere between 12 to 15 months. From Jacks’ letter:

Unvaccinating parent, thanks for screwing up our three-week "vacation" from chemotherapy. Instead of a break, we get to watch for measles symptoms and pray for no fevers (or back to the hospital we go). Thanks for making us cancel our trip to the snow this year. Maggie really wanted to see snow, but we will not risk exposing anyone else. On that note, thanks for exposing 195 children to an illness considered 'eliminated' from the US. Your poor choices don't just effect your child. They affect my family and many more like us.